A. Field of the Invention
This invention relates to blow molding methods for producing hollow containers from softened plastic materials and particularly relates to improved methods of operation for injection blow molding machines.
B. Description of the Prior Art
Certain injection blow molding machine designs have been proposed wherein core assemblies having associated split neck molds are alternately inserted axially into preform injection molds and blow molds to form finished articles. In some of these designs, divergently opening blow mold parts are utilized so that upon separation of relatively movable platens, the blow mold parts open divergently outwardly, thereby allowing removal of the blown article from the blow mold on the retracting core-- the partible neck ring sections being maintained closed about the neck of the blown article. When the platens are fully separated, the finished articles must be removed from the cores. In some of these designs, neck ring parts, which open divergently outwardly, are utilized and these neck ring parts must be opened to allow a stripper ring or other stripping means, for example, a blast of compressed air, to remove the finished article from the core. These designs are disadvantageous because of the requirement for complicated blow mold actuating structure and because of the need for additional elements such as stripper rings or an additional step in the machine cycle to accommodate the air blow off of the article. Other designs have contemplated a machine sequence wherein the blown article remains in the blow mold as the core is retracted. Thus, the closed blow mold acts to strip the finished article from the core. However, in such designs the use of partible neck rings which move only in a direction transverse to the axis of the core have been suggested. These latter mentioned designs are disadvantageous from the view point of machine control and operating speed as, before the core can be retracted, the neck rings must be completely opened to insure that no damage occurs to the neck of the article. In order for the neck rings to be opened, the clamping pressure of the press must first be relaxed to allow the neck rings to move. This results in an increase in machine cycle time and control complexity.
With respect to blow mold actuation, several designs have been proposed wherein common actuating elements have been utilized to open a plurality of blow molds. These designs use two corresponding mold halves, each including a plurality of mating mold cavities. In such designs, the blow molds must be arranged in a linear manner and thus are not adaptable to designs employing annularly alternating blow molds and injection molds. Also, as previously set forth above, it has been known to employ blow molds with divergently outwardly opening mold sections slideably mounted on inclined surfaces or supports. Such blow molds have been utilized with machines employing alternating blow mold and injection mold arrangements. However, commonly in these designs, the mold sections are mounted in cavities in one of the platens and are closed flush with the surface of the platen. This in turn requires the use of an injection press capable of developing very high clamping force to maintain adequate closing pressure between mold parts because the mating contact area between the platens is great, constituting substantially the entire face of each platen. This is disadvantageous for reasons mentioned below and, in addition, this type of blow mold structure is expensive and complicates the problems of heat transfer from the mold.
In previous designs employing rotatable or oscillatable core carrying elements, the supply of various fluids, such as high pressure air and heat transfer mediums, has been accomplished by the use of flexible hoses. A multiplicity of hoses are required to supply the fluids from a stationary distribution point to a plurality of points of use on the rotating core carrying element. This system of supply gives rise to problems of fluid leakage as leaks are prone to occur in the hose couplings and hoses as a result of constant flexing.
Previously, it has been known to mount core assemblies on relatively movable platens of injection molding presses so that the neck ring operating structure and base of the core are received in a cavity in one of the platens. This has in part, been necessary because core assemblies with attendant neck ring actuating structure are large, thereby precluding the mounting of these assemblies on the surface of platens because of the restricted daylight opening between the injection press platens. Thus, the core mounting structure and neck ring actuating structure are recessed below the clamping surface of the platen and access to these parts can be accomplished only by removing the entire core assembly. This unnecessarily complicates maintenance procedures and increases the amount of time necessary to accomplish the change-over of mold parts when a change in container shape is desired. Also, in these designs the neck rings close flush with the face of the platen. This increases the mating surface area between platens. It should be realized that in order to prevent leakage of the injected plastic material between mating portions of the cores and injection cavities, it is necessary to seal such portions at a pressure greater than the pressure of the injected plastic material. Therefore, the greater the surface area of such mating portions, the higher the clamping force of the press platen must be. Thus, machines utilizing such designs require injection presses capable of developing excessively high clamping forces. This is so because the pressure necessary to maintain a seal between the mating portions of the cores and injection cavities to overcome the pressure of the injected plastic material must be generated over substantially the entire face area of the mating platens.